This invention relates to new catalyst compositions useful for initiating and promoting polymerization of alpha-olefins and to a polymerization process employing such composition.
It is well-known that olefins such as ethylene, propylene, and 1-butene can be polymerized in the presence of metallic catalysts, particularly the reaction products of organometallic compounds and transition metal compounds to form substantially linear polymers of relatively high molecular weight. Typically such polymerizations are carried out at relatively low temperatures and pressures.
Among the methods for producing such linear olefin polymers, some of the most widely utilized are those described by Professor Karl Ziegler in U.S. Pat. Nos. 3,113,115 and 3,257,332. In these methods, the catalyst employed is obtained by admixing a compound of a transition metal of Groups IV-B, V-B, VI-B and VIII of Mendeleeve's Periodic Table of Elements with an organometallic compound. Generally the halides, oxyhalides and alkoxides or esters of titanium, vanadium and zirconium are the most widely used transition metal compounds. Common examples of the organometallic compounds include the hydride, alkyls and haloalkyls of aluminum, alkylaluminum halides, Grignard reagents, alkali metal aluminum hydrides, alkali metal borohydrides, alkali metal hydrides, alkaline earth metal hydrides and the like. Usually, the polymerization is carried out in a reaction medium comprising an inert organic liquid, e.g., an aliphatic hydrocarbon and the aforementioned catalyst. One or more olefins may be brought into contact with the reaction medium in any suitable manner, and a molecular weight regulator, such as hydrogen, is often added to the reaction vessel in order to control the molecular weight of the polymers. Such polymerization processes are either carried out at slurry polymerization temperatures (i.e., wherein the resulting polymer is not dissolved in the hydrocarbon reaction medium) or at solution polymerization temperatures (i.e., wherein the temperature is high enough to solubilize the polymer in the reaction medium).
It has been proposed to use organoactinides in place of organotransition metal compounds. It has been shown that given the same ligand array and specific chemical interactions that organoactinides tend to show greater reactivity than early transition metal complexes. Known actinide complexes which catalyze olefin polymerization include chloride salts of uranium and/or thorium or acetylacetonate complexes thereof. They are insoluble in a reaction medium. They afford poor yields and result in product of widely ranging molecular weight. As a result, they have very limited application.
One researcher has reported polymerizing ethylene without added cocatalysts using a uranium bis-pentamethylcyclopentadienyl dihydride dimer complex which is soluble in commonly used reaction medium. However, such a compound would be difficult to use on a commercial basis since it must be used in a vacuum and must be purified in a vacuum. The dihydride catalyst is highly sensitive to and reactive with moisture, impurities and air.